In microscopy, structured illumination is used for depth discrimination in the wide field [1]1 and for enhancing the resolution and the contrast [2]. Generally, a grating or another periodic structure is projected into the specimen [3] or an interference pattern is generated in the specimen by means of interference of coherent component beams [4]. By shifting the illumination structure, images are generated that differ from one another with different phase angles of the period structure. Subsequently, these images are suitably blended with one another so as to obtain an optical sectional image and/or an image with enhanced contrast and enhanced resolution. The disadvantage is that the signal from out-of-focus regions of the specimen is detected as well, which, because of the limited dynamic region of the detector, leads to a reduced signal-to-noise ratio. The strength of the out-of-focus signal limits the useful sample thickness. This is of considerable significance, especially in cases in which the frequency of the structure approaches the diffraction-limited threshold frequency of the optical system, and the contrast of the structure is therefore necessarily low. This invariably applies to cases in which the objective is enhancement of contrast and resolution. 1 Bracketed references refer to the list of references at the end of the specification prior to the claims.
One solution to this problem aims at a partially confocal detection which is made possible by structuring a line of light and detecting the thereby excited fluorescent light by means of a slit detector [5]. However, this method has a number of disadvantages. Structuring occurs only along the line. As a result, the effects of contrast and resolution enhancement are limited to this direction. Thus, especially in cases of nonlinear [6], but also linear, structures [7], the discrepancy between the one direction with enhanced resolution and all other spatial directions is significant. It is necessary to scan the line in a random direction in the specimen plane and to set the phase angle of the period structure. In the prior art, this requires separate actuators for controlling the relative phase angle and the scanning procedure.